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978-1-4244-2332-3/08/$25.00 © 2008 IEEE

A polymeric micro actuator to be integrated into an organic material based lab on chip microsystem

F. Lefevre*, R. Izquierdo Resmiq and Dept. of Computer sciences

Université du Québec à Montréal Montréal, QC, Canada

flefevre@enscpb.fr

S. B. Schougaard NanoQAM and Dept. of chemistry Université du Québec à Montréal

Montréal, QC, Canada

Abstract— The manufacturing of a microvalve actuated by the electroactive polymer polypyrrole has been investigated. The actuator uses the electrochemically induced out-of-plane strain of the polypyrrole/NaDBS (sodium dodecylbenzene sulfonate) system. The manufacturing process of the all polymer (microfluidic channels and actuator) device has been optimized. Deposition of PPY on gold electrodes was achieved even if there is no chemical link between gold and the polymer. Delamination at the gold/PPY interface was observed after the first cycle of actuation. The use of a different electrode material to enable a chemical link between the electrode and the polymer is proposed. This type of micro actuator can easily be integrated in an all organic microsystem. It can also be assembled in series to form an integrated micropump for a disposable device.

INTRODUCTION For the sake of miniaturization, and in a perspective of decreasing the time of analysis, genuine laboratories on chips microsystems have to be created for a variety of fields of applications, such as biology, therapeutic, cytometry, etc. [1] An interesting way for building such a type of microsystems as well as significantly lowering their cost of fabrication is to make them by exclusively employing organic materials [2, 3]. Moreover, in order to facilitate mobility of these microchips, it is advantageous to operate them by only using a low voltage energy source. This outlines why it is very interesting to use conductor polymer and organic materials for such applications [3, 4]. Conductive polymer are typically semiconducting when un-doped, and conducting when doped chemically or electrochemically with donor and acceptor ions [5]. They can be used in the fabrication of light-emitting diodes, chemical sensors and energy storage devices [5-8]. Additionally, it has been proposed that conducting polymer can be used as artificial muscles [9] like actuator in micro valve and micro pumps [10]. The polypyrrole (PPy) is one of polymers used as artificial muscle [11]. Inexpensive, biocompatible, and easy to integrate in a micro system, the polypyrrole is a ideal candidate to be incorporated in a disposable device such as our future all organic lab on chip microsystem.

Belonging to the category of ionic electro-active polymer, the PPy can change its volume resulting from electrochemical electrolyte ion insertion and deinsertion [12]. By applying a current, the number of electrons on the PPy chain can be changed. Then, in order to balance the charge deficiency, ions present in the electrolyte enter or leave the polymer, leading to an in-plane actuation strain. The volume change of the PPy can reach up to 2-3% in strain [13], which corresponds to an anisotropic volume change of 30% with the use of NaDBS (sodium dodecylbenzene sulfonate) as the electrolyte [14-15]. A gold/PPy bilayer structure made on a Si substrate, under a redox current has already been used for the closing of a micro channel [16]. Therefore, polypyrrole could be expected to be used as the actuator for future all organic lab on chip microsystems. Here we present the results on the fabrication and characterization of membrane valves, actuated by a conductive and biocompatible polymer, which can be used for easy large-scale integration into polymer micro fluidic devices. The structure and principle of operation of this device is illustrated on figure 1. This device is composed of two separated parts, which can be manufactured and finally assembled after a plasma treatment. The first part (in the bottom of the device) contains the microchannel for the electrolyte and PPY actuator. The second part (in the top of the device) contains the microchannel for the circulating fluid. This way, the actuation system does not come in contact with the circulating fluid as the latter will circulate in the channels in PDMS. While the polypyrrole has proven to possess impermeable features [17], it is preferable to completely isolate the polypyrrole from the channel, and to solely use its mechanical feature. This device uses the out of plane strain of the PPy in order to block the fluidic channel at the top of it. PDMS elastomer was used for the fluid circulating channel due to its simplicity of manufacturing, biocompatibility, transparency, and its small Young module, which permits deformation by the action of the polypyrrole without cracking [18]. Moreover, the design of the bilayer gold/PPy valves with the PDMS micro channels on the top separated by a thin membrane allows for the possibility of using this type of device like a micro pump. In order to do this various patterns of polypyrrole have to be fabricated close to each other below

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the same microchannel. Then, when inflating and deflating each pattern of polypyrrole alternatively, it is possible to make the fluid move forward in the micro channel in PDMS by a peristaltic effect. This type of micro pump offers affordability, an easy integration, low consumption, is simple to open and has no dead volume [19].

Su8

PDMS

PPY

PDMS micro channel

Activation body

Doped/undoped

Input flow

Fig. 1: Structure and principle of operation of the PPY valve .

MICROSYSTEM FABRICATION In the following section the various photolithography and micro fabrication steps needed for the fabrication of a PPy-micro actuator and microfluidic channel will be described.

1) Microfabrication of the PDMS membrane and channel

This component was fabricated in a three steps process as illustrated on figure 2. The top PDMS component, where the channel is drawn, was made first (right side of figure 2). Then a sheet of PDMS which plays the role of the membrane for the valve was fabricated (left side of figure 2). Finally the two PDMS parts were bonded in order to get the final device (bottom of figure 2). The two first steps were made using similar manufacturing process. A Si (100) wafer was first cleaned with acetone and isopropyl alcohol and heated at 100°C during a night in an oven to remove all moisture and trace of water. As 10 microns deep micro channel are needed SU-8 2015 (MicroChem) photoresist was used in order to define them. PDMS will stick to SU-8 unless the template is silanised. In order to proceed to the silinisation, few drops of tridecafluoro-1,1,2,2-tetrahydrooctyl-1-trichlorosilane (UCT Inc.) were evaporated on a hot-plate in a closed petri dish and the template was exposed to this atmosphere during 6 hours at 80°C. A pre-polymer of PDMS was mixed with a cross-linking agent (kit Silgard 184, Dow Corning) at a 10:1 ratio. The mixture was degassed then spread on the Si/SU-8 samples, and degassed again. The PDMS polymerize at room temperature for two days. At this low temperature of polymerization, PDMS will be more flexible and will have a low shrinkage rate. The cross-linked PDMS of both micro

channel and micro valve membrane samples were then cut and peeled off from the Si substrate. Finally, one sample of PDMS micro valve membrane was bonded with one sample of PDMS micro channel. Bonding of two PDMS parts is usually performed by a plasma treatment, followed by heat treatment. The plasma treatment will incorporate oxygen atoms in the PDMS surface, leaving it hydrophilic during about 15 min. Depending on type of fluid that will be used the microchannel would need to be made hydrophilic. This can be accomplished by polymerizing acrylic acid in the channel.

Si wafer Si wafer

3.Bonding

1. Micro valve fabrication 2. Micro channel fabrication

SU8-8 2015 spread

UV insulation

development

PMDSpolymerisation

Peal off

Fig. 2. Manufacturing process of the PDMS membrane and channel

2) Fabrication of the electrolyte channel and actuation

electrodes

The fabrication of the electrolyte channel and actuation electrode is illustrated on figure 3. A glass substrate of 1 inch square, typically a microscope slide, was used to support the channel and electrode. Substrates were first cleaned by a piranha solution (1/3 H2O2, 2/3 H2SO4). After drying in an oven overnight at 120°C, an adhesion layer of Cr was e-beam evaporated (5 nm, 0.1Å/sec, below 1.10-6 T) to prevent delamination of the gold layer. Then gold was deposited by e-beam evaporation (500 to 1000 Ả at 1Å/sec, below 1.10-6 T). Finally, the pattern of the contacts was drawn by wet etching. The electrolyte channel was made from SU-8 photoresist. In order to obtain channels with 50 microns thickness SU-8 50 (MicroChem) was used. Photoresist was processed at 500 rpm for 10s, then at 2150 rpm for 30s, followed by a 10 min soft bake at 65°C and 30 min at 95°C. Next, the resin was exposed using a mercury lamp (10mW/cm²) for about 90s. Then, the samples were developed (NANOTM SU-8

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developer) to remove the non-exposed resin thus leaving the electrolyte channel. Finally, samples were post baked on a hot-plate for 6 an 20 min at 65°C and 95°C respectively.

Cr+Au E-beam deposition

Wet etching of Cr and Au

SU8-50 deposition

Glass wafer

100µm

100µm

Fig. 3. Fabrication process of the electrolyte channel and actuation electrode.

3) Electrochemical polymerisation of the PPy

The polypyrrole was deposited on top of the gold electrodes by electrochemical polymerization. The gold electrodes and electrolyte channel are shown in figure. Before deposition, the polypyrrole monomer (Aldrich) was passed through aluminum oxide in order to purify it. An aqueous solution of 0.1M pyrrole and 0.1M sodium dodecylbenzene sulfonate (Pfaltz & Bauer) in DI water was freshly prepared. The polypyrrole was electrochemically deposited by chronoamperometry at 0.5V vs. Ag/AgCl and a measured current of approx. 0.1mA/cm². We have fixed the potential rather than the current to prevent over-oxidization of the polymer. By this deposition method, we fix the polymerization time and we can also control the wanted thickness.

4) Assembling of actuator and microchannel

The fabrication of the complete device is accomplished by placing the PDMS micro channel and membrane on top of the electrolyte channel containing the polypyrrole actuator. Alignment of both parts was done using an optical microscope.

RESULTS AND DISCUSSION

PPy was successfully electrochemically disposed on top of the gold electrode as shown on figure 4. As can be seen on this figure the thickness of the deposited PPy can be controlled in order to deposit a 50 microns thick polymer which corresponds exactly to the thickness of the epoxy resin

SU-8 forming the electrolyte channel. This is important as both layers, the SU-8 and PPy, should be at the same level in order to be able to lay down the PDMS part without introducing any pressure at the contact PPy/PDMS channel level when the valve is not actuated.

Fig. 4. SEM image of PPy of a 10 nm thickness, deposited on top of gold

electrode. On the right, the SU-8 resin has a 10 nm thickness. Figure 5 shows an optical micrograph of the actuation area of a completed device. As shown in the figure the microfluidic channel was successfully placed on top of the PPy actuator.

100µm

PPY

Counter electrode

WorkingelectrodeSU-8

PDMS channel

Electrolyte

Fig. 5. View at the top of the completed micro actuator

Finally, the device was mounted into a custom Plexiglas box, organized in such a way that the electrolyte (NaDBS 0.1M) can circulate on the first level and a test liquid can circulate at the PDMS channel level. In order to test the actuation of the PPy, cyclic voltammetry measurements from 0 to -1.5V vs Ag/AgCl were made. The results of those measurements are shown on figure 6. During the first cycle, the reduction peak is situated at -0.85V and the oxidation peak is situated at -0.45V. However, for the second cycle, the reduction and oxidation peaks is displaced to -0.75V and -0.4V respectively. This phenomenon can be explained by the fact that the polymer is compact at the beginning of the test. Then, in order to enter its structure, the sodium ions should drill a passage in the polymer. As this action requires more energy, a higher potential is observed. For the second cycle, as spaces have already been drawn, sodium ions could more easily enter and leave the polymer structure. We also note on figure 6, that the oxidation and reduction peak currents decreased during the second cycle. This is most probably caused by partial delamination between

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gold electrode and the PPy. We have even noticed that after around ten cycles, complete suppression of the peak intensity.

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

-1.5 -1 -0.5 0

Inte

nsity

(A.1

0^-3

)

Voltage (V)

Inte

nsity

(mA

)

Voltage (V)

First cycle

Second cycle

Fig. 6. Cyclic voltamperometry of the PPy in NaDBS 0.1M vs. Ag/AgCl

This simple electrochemical visualization shows that the doping/undoping phenomenon of the PPy is not reversible after several cycles on gold electrodes, most likely due to delamination. It has been demonstrated that roughening the surface of the gold electrode reduced the risk of delamination, for a thin layer of polypyrrole (around 1 micron) [20]. However, no improvement in cyclability of the 50 microns PPy layer was observed when the surface roughness of the gold electrode was increased to 5 nm by wet etching. One of the possible solutions to improve the lifetime of such actuation technology is to intercalate, between the electrode and the PPy, another conductive polymer in which we will have a chemical link between gold and the polymer itself. Another possibility is to completely change the electrode material in order to create a chemical link between the electrode and the conductive polymer. For example, we are presently working on the replacement of gold by carbon electrodes that could be directly manufactured from the pyrolysis of SU-8 photoresist.

CONCLUSION

As part of the fabrication of an “all organic” lab on chip microsystem, a micro valve using the actuation from an electroactive polymer was successfully built. However, delamination problems, which exist between the gold electrode and the polypyrrole actuator, reduce the durability of the fabricated valve. To improve the adhesion between PPy and electrode various alternatives are presently under study. This will allow an considerable increase the reversibility of the polymer inflation cycle and therefore the possibility of using several of these devices in series to fabricate a micro pump based on the peristaltic effect.

ACKNOWLEDGMENT The authors would thank the microfabrication laboratory LMF, Polytechnique de Montréal, where device fabrication

was performed. This research is financially supported by NSERC.

REFERENCES [1] D. Erickson and D. Li, “Integrated microfluidic devices”, Analytica

chimica acta, vol. 507, Issue 1, pp. 11-26, 2004. [2] M. Kock, A. Evans, and A. Brunnschweiler, “Microfluidic Technology

and Applications”, Research Studies Press, Hertfordshire, UK, 2000. [3] H. Becker, and L. E. Locascio, “Polymer microfluidic devices”, Talanta

56, pp. 267-287, 2002. [4] M. Gerard, A. Chaubey, and B.D. Malhotra, “Application of

conducting polymers to biosensors”, Biosensors & bioelectronics, vol. 17, pp. 345-359, 2002.

[5] T. A. Skotheim and J. R. Reynolds, “Conjugated polymers, processing and application”, Handbook of Conducting Polymers third edition, JR Reynolds, 2007.

[6] Q. Pei, G. Yu, C. Zhang, Y. Yang, and A. J. Heeger,” Polymer Light-Emitting Electrochemical-Cells”, Science, vol. 269, pp. 1086-1088, 1995.

[7] J. W. Gardner and P. N. Bartlett, Sensors and Actuators A; physical, “Application of conducting polymer technology in microsystems”, vol. 51, pp. 57-66, 1995.

[8] K.-S., Park, S.B., Schougaard, and J. B. Goodenough, “Composite Conducting-polymer/Iron redox-couple Cathodes for Lithium Secondary Batteries”, Advanced materials, vol. 19, issue 6, pp. 848-851, 2007.

[9] R. H., Baughman, “Conducting polymer artificial muscles”, Synth. Met., vol. 78, pp. 339-353, 1996.

[10] S. A. Wilson, R. P.J. Jourdain, Q. Zhang, R. A. Dorey, Chris R. Bowen, Magnus Willander, Qamar Ul Wahab, M. Willander, S. M. Al-hilli, O. Nur, E. Quandt, C. Johansson, E. Pagounis, M. Kohl, J. Matovic, B. Samel, W. VD Wijngaart, E. W.H. Jager, D. Carlsson, Z. Djinovic, M. Wegener, C. Moldovan, R. Iosub, E. Abad, M. Wendlandt, C. Rusu, and K. Persson, “New materials for micro-scale sensors and actuators An engineering review”, Materials Science and Engineering: R: Reports vol. 56, issues 1-6, pp. 1-129, 2007.

[11] T. Mirfakhrai, J.D.W. Madden, and R.H. Baughman, “Polymer artificial muscles”, Materials Today, vol 10, issue 4, pp. 30-38, 2007.

[12] L. Bay,T. Jacobsen, and S. Skaarup, “Mechanism of Actuation in Conducting Polymers: Osmotic Expansion”, J. Phys. Chem. B, vol. 105 issue 36, pp. 8492 -8497, 2001.

[13] E. Smela, M. Kallenbach, and J. Holdenried, “Electrochemically driven polypyrrole bilayers for moving and positioning bulk micromachined silicon plates” , Microelectromechanical Systems, Journal of Volume 8, Issue 4, pp. 373-383, 1999.

[14] E. Smela and N. Gadegaard, “Volume Change in Polypyrrole Studied by Atomic Force Microscopy”, J. Phys. Chem. B, vol. 105, issue 39, pp. 9395-9405, 2001.

[15] E. Smela and N. Gadegaard, “Surprising volume change in PPy/DBS: an atomic force microscopy study”, Advanced Materials, vol. 11, issue 11, pp. 953-957, 1999.

[16] Y. Berdichevsky andY.-H Lo, ”polymer micro valve based on anisotropic expension of polypyrrole ” , Mat. Res. Soc. Symp. Proc., vol 782, 2004.

[17] Q. Fang, D.G. Chetwynd, J.W. Gardner, C. Toh, and P.N. Barlett, “A preliminary study of conducting polymers as microvalve seals”, Material science and engineering, vol. A355, pp. 62-67, 2003.

[18] J. C. McDonald and G. M. Whitesides, “Poly(dimethylsiloxane) as a Material for Fabricating Microfluidic Devices”, Acc. Chem. Res., vol. 35, issue 7, pp. 491-499, 2002.

[19] J. M. Berg, R. Anderson , M. Anaya , B. Lahlouh , M. Holtz, and T. Dallas, “A two stage discret preistaltic micropump”, Sensor and actuators A: physical, Vol. 104, Issue 1, pp. 6-10, 2003.

[20] Y. Liu, Q. Gan, S. Baig, and E. Smela, “Improving PPy adhesion by surface roughening”, J. Phys. Chem. C.,vol. 111, pp.11329-11338, 2007.

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